1. Field of the Invention
This invention relates to a system employing a satellite for communication between ground stations and, more particularly, to a satellite communication system incorporating an antenna array in which a communicating beam is steered by varying a carrier frequency of the beam.
2. Description of the Related Art
Satellites are now employed for providing communication, such as telephone in land mobile service, between distant points on the surface of the earth. One embodiment of such a system is of considerable interest, namely, an embodiment wherein the satellite travels in a geostationary orbit about the earth. For example, the satellite may be located at a fixed position above the United States. The satellite would carry an antenna having a sufficient beam width in the north-south direction and in the east-west direction to permit the reception and transmission of communication signals between any two points in the United States. The beam width in the north-south direction can be enlarged to include both United States and Canada, if desired. A beam width of approximately 4.5.degree. in the north-south direction is sufficient to cover both Canada and the United States. The beam width in the east-west direction should be approximately 8.degree. to provide the desired coverage. A problem arises in that the use of an antenna having the foregoing beam width in the north-south and east-west directions has less signal gain than is desired. This necessitates larger power amplifiers for driving radiating elements of the antenna.
In previous satellite communication systems, such a wide beam width antenna has employed at least two overlapping beams to provide the coverage, the generation of such beams with a desired overlap requiring the use of separate large reflectors each having a diameter of about 16 feet. In the construction of satellites, however, it is desirable to reduce physical sizes, weights, and power requirements to facilitate the construction and launching of such satellites.